Anyone can make ADRs if you are 180 days or less from the date you want to dine, or 180 + 10 if you have a Disney resort reservation. The dining plan does not enter into it. The only reason you may want to wait until you get a dining plan, or add one before you make reservations, is if you intend to book CRT or a dinner show. If you do not have a dining plan on your reservation at the time you book, you will have to pay in full at booking for these meals, and wait to get refunded until you arrive if you intend to use dining credits for them (unless you are lucky and can cancel the cash reservation and rebook it for the same date and party size with dining plan added).